HARTFORD, Conn. — In fifth grade, Adam Lanza wrote a book that included tales of children being slaughtered and a son shooting his mother in the head.

In the years that followed, he was obsessed with the April 1999 shooting at Columbine High School in Colorado and other mass murders, assembling articles, photos, books, footage and violent video games, including one in which players gun down students in school. He even kept a spreadsheet ranking mass murders.

Nearly a year after Lanza shot his mother to death and then massacred 20 first-graders and six educators at Sandy Hook Elementary in Newtown, Conn., prosecutors closed the case Monday with a report that sketched a chilling portrait of a young man with a twisted fascination with violence.

There was no evidence to explain a motive, Stephen Sedensky III, the state’s attorney for Danbury, said in the report on the Dec. 14 shootings.

The report describes a gunman who had “significant mental health issues” but had sure knowledge of what he was planning: He had materials on mass murder, he smashed his computer hard drive, and he used earplugs during the shooting.

“He had familiarity with and access to firearms and ammunition and an obsession with mass murders, in particular the April 1999 shootings at Columbine High School in Colorado,” according to the report. “Investigators, however, have not discovered any evidence that the shooter voiced or gave any indication to others that he intended to commit such a crime himself.”

Sedensky concluded that Lanza was “solely criminally responsible” for the deaths. It took Lanza less than five minutes to shoot his way into Sandy Hook Elementary School and fire 154 shots from a Bushmaster rifle on his way to gunning down the children and the staff, Sedensky said. Earlier in the morning, Lanza had killed his mother at their home in Newtown. He committed suicide with a handgun as police arrived.

The guns he used in the attack had been purchased legally by his mother, who often took her son shooting and, according to the report, had written out a check to buy him a pistol for Christmas.

“The obvious question that remains is: ‘Why did the shooter murder 27 people, including 20 children?’ Unfortunately, that question may never be answered conclusively,” the report said.

Lanza “was under no extreme emotional disturbance for which there was a reasonable explanation or excuse.”

Sedensky also said there was no clear indication why Lanza chose Sandy Hook Elementary other than that it was close to his home. He attended Sandy Hook from first through fifth grade, but he was never assigned to the classrooms where the shootings took place.

The spiral-bound manuscript that Lanza wrote in the fifth grade at Sandy Hook, “The Big Book of Granny,” was among items seized from Lanza’s home. There is no indication he ever handed the book in at school.

The main character has a gun in her cane and shoots people, and another character likes hurting people, especially children

Until Monday’s report, investigators hadn’t provided details on Lanza’s mental health records, the police response to the shooting or clues to a motive. Sedensky is fighting in court to keep recordings of the 911 calls from being made public.

In his report, Sedensky provides a window into Lanza’s life — describing him as an increasingly isolated young man who cut off ties to his brother and father, would only communicate with his mother by e-mail, and had a fascination with a video game called “Dance Dance Revolution.” In fact, Lanza was a regular at a local theater nearly every weekend in 2011 and early 2012 just to play the dance game, for up to 10 hours at a time.

Lanza refused medication and behavioral therapies suggested to him in order to treat tendencies consistent with Asperger’s disorder, obsessive-compulsive disorder symptoms and other sensory issues.

A timeline released with the report indicates that nearly six minutes passed between the arrival of the first Newtown police office and the time officers entered the school. The report said officers were operating under the belief there may have been more than one shooter.

Whether the delay made any difference was unclear. The report said Lanza killed himself about a minute after the first officer arrived.

To try to figure out a motive, investigators said, they interviewed members of Lanza’s family — his father and brother cooperated fully — along with teachers and others. They said they also tried within the limits of privacy laws to gather information on his medical treatment.

Donna Soto, the mother of slain teacher Victoria Soto, said in a statement that nothing could make sense of the shooting.

“Yes, we have read the report. No, we cannot make sense of why it happened. We don’t know if anyone ever will,” Soto wrote. “We don’t know if we will ever be whole again, we don’t know if we will go a day without pain, we don’t know if anything will ever make sense again.”

Sedensky’s report is a summary of a much larger Connecticut state police evidence file that is expected to be released at a later date.